Tryptophan hydroxylation in cockroach (Periplaneta americana) nervous tissue was measured and compared to the hydroxylation of tryptophan in rat brain. Tryptophan hydroxylation in both tissues requires a pterine cofactor, and is inhibited by p-chlorophenylalanine. The molecular weight of the protein responsible for hydroxylation of tryptophan in cockroach nervous tissue obtained from gel filtration was estimated to be 54,000. The pH optima and enzyme kinetics differed greatly between the two hydroxylases. Hydroxylation of tryptophan by the enzyme obtained from cockroach tissues incubated with dimethyltetrahydropterine had a pH optimum of about 5.8-5.9 and a K(m) in crude enzyme preparations of 2.6 x 10(?6) M and is activity was substrate inhibited above 10(?4) M tryptophan. Hydroxylation of tryptophan by the enzyme obtained from rat brain incubated with dimethyltetrahydropterine had a pH optimum of about 6.5-7.0, a K(m) of about 6.7 x 10(?4) M and exhibited no substrate inhibition at tryptophan concentrations up to 2 x 10(?3) M. When incubated with biopterin, the presumed natural cofactor, the hydroxylase from cockroach tissues had a K(m) of about 6.8 x 10(?5) M and no substrate inhibition occurred at tryptophan concentrations up to 2 x 10(?3) M. Under the same conditions rat hydroxylase had a K(m) of 1.1 x 10(?5)M and substrate inhibition occurred above 10(?4) M tryptophan. Unlike the mammalian situation, administration of tryptophan peripherally did not change the 5-hydroxytryptamine concentration in cockroach nervous tissue, but did increase tryptophan levels. The low V(max) values of the cockroach hydroxylase and the inability of administered tryptophan to elevate 5-hydroxytryptamine levels suggest that in the cockroach hydroxylation of tryptophan itself may be the limiting factor in the biosynthesis of 5-hydroxytryptamine.